The present invention relates to moisture sensors, and more particularly to a device and method for determining the moisture content of a paper sheet by measuring the transmission of various wavelengths of infrared radiation through the sheet. The measurement of the moisture content of the sheet is compensated for changes in the infrared absorption characteristics of water caused by changes in sheet temperature.
In the commercial manufacture of paper, paper is produced in a continuously moving sheet called a "web". Since the web is produced from an aqueous suspension including wood pulp fibers, cotton fibers and various chemicals, the web initially contains a considerable amount of moisture. Most of this moisture is removed during the paper making process. However, for a variety of reasons, it is desirable to maintain at least some moisture in the web. For example, if the web is too dry, it will tend to curl at the edges.
Paper web is normally dried by passing it around heated steel drying drums. However, this technique tends to dry the web unevenly. Such uneven drying produces paper of uneven quality. Hence, various devices have been developed to controllably moisten or dry only portions of the web, after the web has passed around the drying drums, to thereby produce a web having a uniform moisture content. Obviously, the paper mill operator, or the paper mill's process control computer, must know the moisture profile of the web before these moistening and drying devices can be used effectively. Web moisture sensors have therefore been developed which scan back and forth across the width of the web to determine its moisture content at various locations.
Water absorbs infrared radiation. Certain types of web moisture sensors take advantage of this phenomenon by directing a beam of infrared radiation at a web and measuring the intensity of the infrared beam after it passes through the web. The more moisture in a web, the greater the absorption of the infrared radiation.
Some of these infrared moisture sensors utilize two infrared detectors with an infrared band pass filter positioned in front of each detector. The pass band of each filter is chosen so that each detector receives energy only in a narrow region of the infrared spectrum. One filter is chosen to pass infrared radiation in a region of strong absorption by the water in the web. Thus, the detector associated with this filter is sensitive primarily to the amount of water in the web. This first detector receives more infrared radiation when the web is dry and less infrared radiation when the web is moist.
The band pass filter associated with the second detector is selected to pass infrared radiation in a portion of the infrared spectrum where there is relatively little absorption by moisture. In this second portion of the infrared spectrum, most of the infrared absorption is due to the web fibers themselves, not to the moisture in the web. Therefore, as the web fiber weight per unit area (i.e. the "basis weight") increases, this second detector receives less infrared radiation. The output of this second detector can thus be used to compensate the moisture measurements of the first detector for changes in the basis weight of the sheet. When the outputs from these two detectors are properly combined, these types of moisture sensors provide a measurement of the amount of moisture contained in the web or the percentage of moisture in the web, so that the moisture measurement is unaffected by changes in the basis weight of the sheet.
The intensity of the transmitted infrared beam is, however, not only dependent upon the moisture content of the web and its basis weight. The absorption of infrared radiation by the moist web also varies with the wavelength of the infrared rays. The water and web fibers absorb certain wavelengths of the infrared spectrum more effectively than other wavelengths so that there exists absorption peaks and valleys at various wavelengths along the infrared absorption spectrum. Moreover, these peaks and valleys shift to shorter wavelengths with increases in web temperature and to longer wavelengths with decreases in web temperature. However, the previously described infrared moisture measuring devices fail to compensate for shifts in the infrared absorption spectrum with changes in web temperature. Therefore, the moisture measurements of these devices are subject to significant error.